Fetching remote data from the REST service works, basic drawing too.

That's a first prototype.

src/App.js

@@ -1,16 +1,21 @@
-import React from 'react';
+import React, {useState} from 'react';
 import Maze from "./Maze";
-import {maze1, maze2, maze3} from "./testdata";
+import InputForm from "./InputForm";
 
-export default function Square() {
+export default function App() {
-    const mazes = [{grid: [[]]}, maze1, maze2, maze3];
+    const [maze, setMaze] = useState({});
-    const renderedMazes = mazes.map(maze => (<div>
+    let title;
-        <h1>The Maze ({maze.width}x{maze.height}).</h1>
+    if (!!maze.grid) {
-        <Maze labyrinth={maze}/>
+        title = <h1>The Maze ({maze.width}x{maze.height})</h1>;
-    </div>))
+    } else {
+        title = <span/>;
+    }
     return (
         <div>
-            {renderedMazes}
+            <InputForm handleResult={setMaze}/>
+            {title}
+            <Maze labyrinth={maze}
+                  showSolution={true}/>
         </div>
     );
 }

src/Cell.js

@@ -1,15 +1,14 @@
 import React from 'react';
 
-export default function Cell({spec, rowIndex, cellIndex}) {
+export default function Cell({spec, rowIndex, cellIndex, showSolution}) {
     let classes = "cell r" + rowIndex + " c" + cellIndex;
     if (spec.top) classes += " top";
     if (spec.right) classes += " right";
     if (spec.bottom) classes += " bottom";
     if (spec.left) classes += " left";
-    if (spec.solution) classes += " solution";
+    if (spec.solution && showSolution) classes += " solution";
     if (spec.user) classes += " user";
     return (
-        <div className={classes}>
+        <div className={classes}/>
-        </div>
     );
 }

src/InputForm.js

@@ -0,0 +1,111 @@
+import React, {useState} from 'react';
+import ValidatingInputNumberField from "./ValidatingInputNumberField";
+
+export default function InputForm({handleResult}) {
+    const [width, setWidth] = useState(2);
+    const [height, setHeight] = useState(2);
+    const [id, setId] = useState(null);
+    const [status, setStatus] = useState("typing"); // "typing", "submitting"
+
+    if (status === "submitted") {
+        return <span/>;
+    }
+    const callAPI = () => {
+        let url = "https://manuel.friedli.info/labyrinth/create/json?w=" + width +
+            "&h=" + height;
+        if (!!id) {
+            url += "&id=" + id;
+        }
+        fetch(url)
+            .then(response => response.json())
+            .then(result => {
+                handleResult(result);
+                // FIXME doesn't update the contents of the text input field.
+                setId(_ => result.id);
+            })
+            .catch(reason => {
+                console.error("Failed to fetch maze data.", reason);
+                // FIXME alert is not user friendly
+                alert("Failed to fetch maze data: " + reason);
+            })
+            .finally(() => {
+                setStatus("typing");
+            });
+    };
+    const handleSubmit = (e) => {
+        e.preventDefault();
+        setStatus("submitting");
+        callAPI();
+    };
+    const validateWidthHeightInput = value => {
+        if (isNaN(value) || "" === value || (Math.floor(value) !== Number(value))) {
+            return {
+                valid: false,
+                message: "Must be an integer greater than 1.",
+                value
+            };
+        }
+        if (value < 1) {
+            return {
+                valid: false,
+                message: "Must be greater than 1.",
+                value
+            };
+        }
+        return {
+            valid: true,
+            value
+        };
+    };
+    const validateIdInput = value => {
+        // FIXME doesn't handle strings with characters correctly (e.g. "asdf" yields an empty value, due to "type=number").
+        if (isNaN(value) || ("" !== value && ((Math.floor(value) !== Number(value))))) {
+            return {
+                valid: false,
+                message: "Must be empty or an integer",
+                value
+            };
+        }
+        return {
+            valid: true,
+            value
+        }
+    };
+    return (
+        <form onSubmit={handleSubmit}>
+            <ValidatingInputNumberField id={"width"}
+                                        label={"Width"}
+                                        defaultValue={width}
+                                        constraints={{
+                                            min: 2
+                                        }}
+                                        validatorFn={validateWidthHeightInput}
+                                        disabled={status === "submitting"}
+                                        onChange={setWidth}
+            /><br/>
+            <ValidatingInputNumberField id={"height"}
+                                        label={"Height"}
+                                        defaultValue={height}
+                                        constraints={{
+                                            min: 2
+                                        }}
+                                        validatorFn={validateWidthHeightInput}
+                                        disabled={status === "submitting"}
+                                        onChange={setHeight}
+            /><br/>
+            <ValidatingInputNumberField id={"id"}
+                                        label={"ID (optional)"}
+                                        defaultValue={id}
+                                        validatorFn={validateIdInput}
+                                        disabled={status === "submitting"}
+                                        onChange={setId}
+            /><br/>
+            <button type={"submit"}
+                    disabled={status === "submitting"
+                        || isNaN(width)
+                        || isNaN(height)
+                    }>GO!
+            </button>
+        </form>
+    );
+}

src/Maze.js

@@ -2,9 +2,14 @@ import React from 'react';
 import Row from "./Row";
 
 
-export default function Maze({labyrinth}) {
+export default function Maze({labyrinth, showSolution = false}) {
+    if (!labyrinth.grid) {
+        return <div>No valid maze.</div>
+    }
+
     const maze = labyrinth.grid.map((row, rowIdx) => <Row key={"r" + rowIdx} spec={row}
-                                                          index={rowIdx}/>);
+                                                          index={rowIdx}
+                                                          showSolution={showSolution}/>);
     return (
         <div className={"maze"}>
             {maze}

src/Row.js

@@ -1,11 +1,12 @@
 import React from 'react';
 import Cell from "./Cell";
 
-export default function Row({spec, index}) {
+export default function Row({spec, index, showSolution}) {
     const cells = spec.map((cell, cellIdx) => <Cell key={"c" + index + "-" + cellIdx}
                                                     spec={cell}
                                                     rowIndex={index}
-                                                    cellIndex={cellIdx}/>)
+                                                    cellIndex={cellIdx}
+                                                    showSolution={showSolution}/>)
     return (
         <div className={"row"}>
             {cells}

src/ValidatingInputNumberField.js

@@ -0,0 +1,49 @@
+import React, {useState} from 'react';
+
+export default function ValidatingInputNumberField({
+                                                       id,
+                                                       label,
+                                                       defaultValue = 0,
+                                                       constraints = {},
+                                                       validatorFn = (value) => {
+                                                           return {valid: true, value};
+                                                       },
+                                                       disabled = false,
+                                                       onChange = _ => {
+                                                       }
+                                                   }) {
+    const [error, setError] = useState(null);
+    const [value, setValue] = useState(defaultValue);
+
+    const handleValueChange = (e) => {
+        const value = e.target.value;
+        const validation = validatorFn(value);
+        if (!validation.valid) {
+            setError(validation.message);
+        } else {
+            setError(null);
+            onChange(validation.value);
+        }
+        setValue(validation.value);
+    };
+    let errorComponent;
+    if (!!error) {
+        errorComponent = <span>{error}</span>;
+    } else {
+        errorComponent = <span/>;
+    }
+    return (
+        <span>
+            <label htmlFor={id}>{label}: </label>
+            <input id={id}
+                   type={"number"}
+                   onChange={handleValueChange}
+                   value={value || ""}
+                   min={constraints.min || null}
+                   max={constraints.max || null}
+                   disabled={disabled}
+            />
+            {errorComponent}
+        </span>
+    );
+}

src/styles.css

@@ -42,6 +42,9 @@ div.cell.left {
     border-left-color: #000;
 }
 
+input:invalid {
+    border-color: #f00;
+}
 body {
     font-family: sans-serif;
     margin: 20px;